Biomass fast pyrolysis involves the rapid heating of biomass in an inert atmosphere to produce bio-oil, a complex chemical mixture with a variety of commercial applications, including production of renewable materials from aromatic hydrocarbons.
Fast pyrolysis is often accompanied by catalytic upgrading of pyrolysis products (including bio-oil and vapors), which provides a highly deoxygenated product stream consisting mainly of hydrocarbons.
A variety of heterogeneous catalysts have been used or proposed for use in catalytic upgrading of the pyrolysis product stream, including synthetic zeolites. Synthetic zeolites are crystalline aluminosilicates, having the main formula M2/nO.Al2O3.xSiO2.yH2O. Their building blocks are SiO4 and Al tetrahedra linked at their corners by a common oxygen atom to provide an inorganic macromolecule with a unique three dimensional framework. Synthetic zeolites are typically classified by pore size as either microporous (pore diameter of less than about 2 nm) or mesoporous (pores diameter ranging from about 2 nm to about 50 nm).
Among the synthetic zeolites proposed for use in catalytic upgrading is Zeolite Socony Mobil-5 (“ZSM-5”). ZSM-5 was first prepared in 1975 and is still in widespread use in the petroleum industry for interconversion of hydrocarbons. It is a microporous Mordenite framework inverted structure (MFI) zeolite with a median pore size of about 0.55 nm. While ZSM-5 has demonstrated good aromatic yields in catalytic upgrading of the pyrolysis products, many intermediate and primary products of pyrolysis remain unreacted given pore sizes.
Other synthetic zeolite catalysts have been proposed for use in catalyatic vapor upgrading, including catalysts with larger pores or mesoprous characteristics, but have generally proven inferior.
For example, mesoporous zeolite catalysts (e.g., A1-MCM-41, A1-MSU-S and alumina stabilized Ceria) have been proposed, but their pore sizes have proven too large or they've lacked the shape selectivity or acidity of ZSM-5. They are also less hydrothermally stable than ZSM-5.
Hydrothermally stable mesoporous aluminosilicates prepared from zeolite seeds (known as MSU-S) have also been proposed. Yet, MSU-S have shown low selectivity for aromatics production compared to ZSM-5.
Accordingly, there remains a need for new catalysts for use catalytic upgrading of biomass pyrolysis products and more particularly, catalysts that exhibit (i) high selectivity toward particular aromatics, such as C6-C9 monoaromatics, particularly p-xylene, (ii) high reaction rates and (iiii) low rates of deactivation, e.g., by char production.
In addition, there remains a need for improved processes for preparing BTEX (an acronym that stands for benzene, toluene, ethylbenzene, and xylenes) from isobutylene derived from various biologically derived feedstocks.